Apparatus for blow molding

ABSTRACT

This invention relates to apparatus for molding and, more particularly, to apparatus for the fabrication of hollow bodies from thermo-plastic material wherein, during a first fabrication step, the piece is pre-fabricated and, in a second step, the piece is positioned within a mold to form the hollow body by blowing until the hollow body reaches the desired shape.

1 8* Unite States atent [1 1 [111 Mehnert Aug. 7, 1973 1 APPARATUS FORBLOW MOLDING 2,908,034 10/1959 Hac kctt 425/387 X 96 W 4 6 1 JohannesMendcnuber 32335335 111963 mifnale 416 Swburg, Germany 3,496,597 2/1970Ayres 425/387 73 Assignee; Siemag si Maschinenbau 3,503,099 3/1970 Uhlrg425/249 Gesellschatt mit beschrankter v naftungnundesrepublik, GermanyPrzrnary Examzner-J. Spencer Overholser a a -7 v AssistantExammerMlchael 0. Sutton {22] Flled: 1972 AttorneyNorman S. Blodgett 21Appl. No.: 233,437

- 57 ABSTRACT Related US. Application Data 63 This invention relates toapparatus for molding and, 1 ggg s 'g of more particularly, to apparatusfor the fabrication of a an one hollow bodies from thermo-plasticmaterial wherein,

during a first fabrication step, the piece is pre- 425/326 3;??2'58;fabricated and, in a second step, the piece is positioned [58] Fie'ld BB within a mold to form the hollow body by blowing until I I the hollowbody reaches the desired shape. 6] References Cited Claims, 7 DrawingFigures UNITED STATES PATENTS 2,657,431 11/1953 Slaughter 425/326 -X2,936,481 5/1960 Wilkalis et a1. 425/249 27a 1' s t PATENTED 3.751.207

4 3c 2 1%; Wm

FIG. 7

APPARATUS FOR BLOW MOLDING CROSS-REFERENCE TO RELATED APPLICATIONS Thisapplication is a continuation-in-part of patent application Ser. No.862,796 filed Oct. 1,1969, now abandoned. 1

BACKGROUND OF THE INVENTION In the molding of hollow articles of'thermo-plastic materials, the two well-known fabrication procedures arecalled extrusion blow molding. and injection blow molding. The extrusionblow molding method starts with an extruded tube (parison) and the baseof the article is formed by clamping off a section of the tube with themold (which usually consists of two parts). The separated piece isthereafter expanded within the mold in order to form a hollow body whichis complementary to the mold. A disadvantage of this procedure consistsof the fact that the squeezing off of the raw piece causes materialprotrusions to be formed within this area; these must be removed duringor after the fabrication process. This results notonly in loss of thematerial but also in an additional expenditure for the work of remedyingthe flow. Another difficulty experienced in extrusion blow molding isthat, in order to form the parison, ther is a large machiningexpenditure involved in providing a suitable nozzle. At the same time,irregularities occur during the fabrication; these irregularities can bethe result of the devices for holding the nozzles or can occur becauseof the conditions occuring during the flow around the air-injectionneedle. In the case of vertical extrusions, there is the danger of undueextension or swinging of the tube and, in the case of horizontalextrusions, a sagging of the tube cannot be prevented. In addition, thisprocedure requires devices for the welding of seamlines and the guidingof the bar of the tube. A further disadvantage is found in that thefabrication of the tube is basically a continuous procedure, while theblowing procedure and the separation of the raw piece are of adiscontinuous nature, which fact makes it necessary to coordinate thesetwo different moving procedures.

The injection blow molding method, on the other hand, avoids a number ofthe disadvantages set forth above but has its own weaknesses. Forexample, a relatively high pressure is necessary to press the plasticmaterial through the narrow nozzle into the injection molding recess.The high pressure presents difficulties in centering the pressure airneedle which is inserted into the molding, particularly when this bar isthin and that the surface temperatures of the raw piece is not allowedto become too high, otherwise the material will stick when the surfacesare removed. On the other hand, if the raw piece has a cool surfaceblowing becomes difficult. Therefore, a compromise has to be made byadjusting the surface temperature and very careful workmanship isrequired. These and other difficulties experienced with prior artapparatus have been obviated in 'a novel manner by the presentinvention.

It is therefore, an outstanding object of the invention to provideapparatus for molding which eliminates the disadvantages of extrusionblow molding and injection blow molding, while'at the same timecombining the advantages of these two procedures.

Another object of this invention is the provision of apparatus formolding which eliminates machine work in the case of extrusion blowmolding and the forming of unwantd protrusions on the formed article.

A further object of the present invention is the provision of apparatusfor molding in which the operating procedures are simplified and thewall thickness of the parison and of the final formed article is readilycontrollable.

It is another object of the instant invention to provide apparatus formolding in which waste ,of materials and trimming work are virtuallyeliminated. 7 With these and other objects in view, as will be apparentto those skilled in the art, the invention resides in the combination ofparts set forth in the specification and covered by the claims appendedhereto.

SUMMARY OF THE INVENTION molding having an extruder with an outlet boreand a long. It has been proven, as a practical matter, that the Irelationship of the length of the bar to its diametercannot be greaterthan a ratio of 2.5/1 if a dislocation of the free end of the bar is tobe avoided. It is necessary to prevent such a dislocation of the freeend of the bar, because a small variation in location of the bar withinthe recess can lead to a blowup of the raw material during the blowingprocedure. It is possible to center long and thin air needles byproviding additional support elements, but such a procedure (which,theoretically, permits the use-of longer bars) calls for the use ofdifficult procedures and complex devices to prevent the sagging of thebar. In the use of the injection blow molding process, it is extremelycritical to coordinate and control the injection pressure, temperaturedistribution, and other factors. It is particularly important moldhaving a recess aligned with the bore. A blow rod extends-through themold and means is provided bringing about relative movement between therod and the bore from a first position in which the end of the rod liesin the bore to a second position in which the end lies in the recess.

BRIEF DESCRIPTION OF THE DRAWINGS The character of the invention,however, may be best as illustrated by the accompanying drawings, inwhich:

FIG. 1 is a vertical sectional view of apparatus embodying theprinciples of the present invention with the apparatus shown in a firstcondition,

FIG. 2 is a sectional view of the apparatus shown in a second condition;

FIG. 3 is a sectional view of the apparatus shown in a third condition,

FIG. 4 is a vertical sectional view of a modified form of the apparatus;

FIG. 5 is a sectional view of another modification of the apparatus,

FIG. 6 is a vertical sectional view of another modified form of theinvention, and

FIG. 7 is a vertical sectional view of another modification of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention solves theproblems of the prior art by using a procedure in which the blowing rodis introduced into a chamber filled with the thermoplastic material. Arelative movement between the rod and an exit opening of the chambercauses a tube of the plastic to surround the rod and to be carried intoa mold. After the tube has been pulled out in this manner, the plasticin the chamber and the remaining plastic in the mold are separated by acutting operation. Preferably, the chamber contains the plastic inamolten condition with sufficient viscosity to preventit from leaving thechamber during the introduction of the air rod. The shape of the openingto the chamber as well as the length of the opening within a relativelythick chamber wall determins the outside dimensions of the tube; theinside dimension is determined by the size of the air rod. In this way,the necessity for the high pressure used in injection blow molding isdone away with and the provision ofa special nozzle for the productionof a tube (as used in injecting blow molding) is eliminated. At the sametime, under ideal conditions, the withdrawal of the air rod comparesfavorably with the extrusion of a tube, because the inside wall of theopening is the only frictional surface and practically no relativemovement exists between the blow rod and the surrounding plastic. Theseparation of the raw piece at a position where the stream of plastic issolid permits a discontinuous extrusion procedure to take place withoutimpairing the raw pieces which follow. The opportunity to center theblow rod by a holding device positioned outside of the chamber preventsany nonuniformity ofthe plastic tube surrounding the blow rod, as isoften created by the nozzle of the known extrusion devices.

Referring first to FIG. I, wherein are best shown the general featuresof the invention, the blow rod 1 is shown as fastened to a piston rod 2.A piston 3 is mounted on the piston rod and is movable within a cylinder4. This cylinder brings about movement of the air rod 1 by means ofpressure air supplied from a controlled source in the manner well-knownin the art. Between the blow rod 1 and the piston rod 2 is mounted aflange 7. The blow rod 1 is pressed into the opening 5 ofa chamber 6 inan extruder until the flange 7 closes off the opening 5, as best shownin FIG. 1.

With the opening 5 closed by the flange 7, a pressure is created in thechamber 6 by the operation of a feed means such as a screw- 13. At thesame time that the pressure is created within the chamber, the piston 4is moved away from the chamber, so that the air rod 1 together with asurrounding tube 8 of plastic is withdrawn from the chamber openinguntil a second position is reached, as shown in FIG. 2.

A blow mold 10 is prrovided consisting of two parts which come togetherto form the main cavity. The raw piece is cut off while still solid atthe point 9, as shown in FIG. 3. Compressed air is introduced into theblow rod 1 through air passages 11 and 12 and this expands the raw tube8 and presses the material against the inner walls of the cavity in themold as shown in FIG. 3.

In the form of the invention shown in FIGS. 1, 2, and 3, the strippingof the formed article is accomplished after swinging the air rod and thecylinder around the vertical axis indicated by the line A-A and shown indotted lines. The swinging movement and the stripping action whichfollows are not shown in detail in the drawings.

A connecting means such as a cylinder 24a is joined to the screw 13 sothat it may be retracted from the position shown in FIG. 1 to theposition shown in FIG. 2.

The cylinder 24a is connected to an air pressure tank, not shown in thedrawing, through lines 25a and 26a. From this air pressure tank thecylinder 24a may be supplied with air pressure to bring about therequired movement of the screw 13. Between the supply line 25a (thoughwhich the cylinder is supplied with pressure to bring about the movementof the conveyor 13 from the position of FIG. 1 to the position of FIG.2) and the cylinder 24a is located a valve 27a which controls the supplyof air pressure in accordance with the movement of the blow rod 1. Forthis purpose, the piston 3 is equipped with an additional piston rod 28aextending in the opposite direction to the piston rod 2, the piston rod28a passing out of the cylinder. The piston rod 28a carries an arm 29ato which is fastened a cable 300. This cable passes over a guide pulley31a and is wound around a drum 320 with a plurality of windings. Thedrum 32a is rotatably supported on its ends and is connected to a spiralspring, not shown by which means the drum 32a is rotatably biased in thedirection opposite to the arrow 33a. In this way the drum 32a acts toroll up the cable 301;. On the drum 32a is fastened a track 34; this isdone by gluing or the like, depending on the type of track and itsgradiant. Furthermore, there is located adjacent the drum 320 areflex-photocell 35a through which the track 34 may be scanned. Thisphotocell is connected to an amplifier 360 which increases the signalproduced by the photocell and transfers the signal to the valve 27a forits control. As soon as the blow mod 1 leaves the position shown in FIG.1, the valve 270 is informed of the movement of the piston 3 by means ofa switch, not shown, so that air pressure acts on the cylinder 24a. Theamount of air pressure is selected in such a way that the screw 13 movesrelatively slowly out of the position shown in FIG. 1 and into theposition shown in FIG. 2. Through the movement of the piston 3 and withit the movement of the blow rod 1, the drum 32a is turned in thedirection of arrow 330 by the cable 30. As soon as the track 34a isrecognized by the photocell 350, the cell sends a signal too theamplifier 36a which, in turn, sends a signal to the valve 27a as aswitch-off impulse. By a suitable selection of the shape of the track340, the advance of the screw 13 can be coordinated with the movement ofthe piston 3 and the air rod 1.

FIG. 4 shows a simple modification of the invention in which thestripping of the formed article is brought about by an axial movement,so that no swinging or turning movement has to be made. For thisoperation it is only necessary that the cylinder 24 and the piston 23 belocated a sufficient distance from the chamber opening, so that, afterthe opening of the mold 30, the air rod 21 may be pulled by the pistonrod 22 even further back to a third position, while a stripper l4 pullsoff the formed article fromthe blow rod 21.

The stripper has the shape of a ring and its inside diameter is largerthan the outside diameter of the flange, so that the flange 27 may gothrough the stripper 14 without interference. On the other hand, theinside diameter is small enough so that the formed article 28 willstrike it and be removed from the air rod. Ater stripping, the air rodis brought back into a position similar to that shown in FIG. 1, so thatthe working cycle may be repeated. The extruder has a screw 33 forproducingpressure on the molten plastic in the chamber 26 to press itthrough the opening 25 as before. The

flow of plastic is cut off at the point 29 and the air passages 31 and32 provide the pressure air for blowing.

FIG. 5 shows a variation of the invention in which an air rod 41 isstationary and the opening in the chamber 46 is formed by a slidablesleeve 45a, which is-movable within the bore 45b of the chamber. Piniongears 55 and 55' engage a rack 56 and 56' formed on the outside surfaceof the sleeve, and the sleeve can be pulled over the air rod 41 to aposition indicated by dotted lines until the sleeve rests against theflange 47. When the pressure is created by the worm gear 53 within thechamber 46, the space formed by the air rod 41, the inside of the sleeveand the flange 47 is filled up completely with plastic material. Afterthat, the sleeve 55a is returned to the housing to the position shown infull lines. The air rod 41 and cylinder 44 are then swung about a hingepin 57 into a position where the plastic is engaged by the mold 50moving in to allow the blow forming of the article within the moldrecess by pressure air operating through passages 51, 52 in the hinge57, piston 43, rod 42, and the air rod 41. Stripping of Q the finishedarticle 48 is accomplished by the ring 54.

In the version of the invention shown in FIG. 6, the movement of thetransport screw 13b is controlled electrically in accordance with themovement of the blow rod 1b. In this arrangement, the piston 3b isequipped with a gear rack 41 formed on the side of piston rod 2 whichengages a gear 42 which is fixedly supported. This gear 42 is connectedto a contact 43, which, during turning of the gear 42, slides along onarcuate potentiometer 44. To the potentiometer 44 and the contact 43 isconnected an electrical control device 45 of known design from which isfed constant voltage durect-current electricity to the optentiometer 44.The contact 43 is caused to swing by the gear 41 and delivers a voltagesignal which indicates at all times the true position of the gear rack41 and, therefore, the position of the piston 3. Through the controldevice 45 this voltage signal is used for the on-and-off switching of anelectric motor 46, which carries a gear 48 on its output shaft 47. Thegear wwheel 48 engages a rack 49-which is mechanically connected to thescrew 13 (as indicated by the dotted line) and so activates the axialdisplacement of the screw 13 in accordance with the position of thepiston 3.

In the variation of the invention shown in FIG. 7 the piston 3c isprovided with a piston rod 51 which extends through the end of thecylinder. This rod 51 acts on a valve 52 which is supplied with fluidpressure medium from a hydraulic pump 53. When the piston moves towardsthe left (from the position shown in FIG. 7), the valve 52 will beactuated by the rod 51 in such a way that the pressure fluid streamflowing through the pipe line 54 to the valve 52 reaches a piston valve56. It switches the latter in such a way that the pressure fluid is ableto flow to a piston-cylinder-unit 59 through the pipe line 57, thepiston valve 56, and the pipe line 58. In this way the screw 130 isdisplaced axially in the direction of the mold 100. This displacement ofthe transport conveyor 13c is made in accordance with the movement ofthe piston 30. When the piston 30 moves back into the position shown'inFIG. 7, the valve 52 is switched by the rod 51 in the opposite directionand the fluid stream is now able to flow through pipe line 54, valve 52,and pipe line 60 to the piston 6 valve 56. This piston valve is nowswitchedin such a way that the hydraulic stream may flow to the otherside of the piston through pipe line 57, valve 56, and a pipe line 61.The transport conveyor 13 is in this way returned to its startingposition.

The present invention has been described in a general way above. It maybe practical in certain operations to use several air rods for a givensindle mold. During the blowing operation within the mold, the other airrod or rods may be brought to a desired temperature, or may be dippedinto an opening in the'melting chamber to be pre-warmed before pullingout a new raw tube. The melting chamber can be equipped with a number ofopenings, each of which can be provided with its own air rod. Theseopenings could be arranged side-by-side or one above the other. Also,the fabrication of a complex article may be brought about in thesimplest way, because the air rod mayrecieve, on the one hand, desiredcross-section ratios and, on the other hand, may bring about, at thesame time, with exceptional accuracy, the wall thickness distributiondetermined by the melting pressure and the flow velocity. All movementsmay be made with relatively small force because there is only the smallresistance due to the flow of material to be overcome. An additionaladvantage of the "invention consists in the fact that the air supplychannels do not have to be dimensioned so closely becuase of therelatively small pressure within the melting chamber. There is no dangerof the plugging of channels occurring because of metal present withinthe chamber.

Another important advantage of this invention is that the wall thicknesscan be determined by the retraction speed of the air rod, the pressurecreated within the chamber, and the viscosity of the molten plastic.This is in comparison with normal extrusion blowing procedure wherechanges of wall thickness of the raw parison may be made only by anadjustment of the relative sizes of the nozzle ring and core. Finally,the separation of the raw parison with a cutting device can be replacednow by adjusting the pressure within the chamber to a negative pressureand separation occurs automatically after the air rod has beencompletely pulledout. It will be understood that the closing of the moldcan be brought about by a control apparatus which, at the same time,lowers the chamber pressure to cut off any movement of plastic throughan opening. The mold, the flange, and the air rod, of course, areprovided with continuous air passages for the blowing of the parison.The mold is also provided with recesses into which the flange is clampedwhen the mold is closed. This also brings about the arrangement ofcontinuous passage of air into the main mold recess. I

It is obvious that minor changes may be made in the form andconstruction of the invention without departing from the material spiritthereof. It is not, however, desired to confine the invention to theexact form herein shown and described, but it is desired to include allsuch as properly come within the scope claimed.

The invention having been thus described, what is claimed as new'anddesired to secure Letters Patent it:

1. Apparatus for molding, comprising a. an extruder having an outletbore and a feed means, i

b. a mold having a cavity aligned with the bore,

c. a blow rod extending through the mold, and

d. an actuator for bringing about relative movement between the rod andbore from a first position in which the rod lies in the bore to a secondposition in which the rod lies in the cavity, and

e. means connecting the actuator and the feed means so that the latteris operated to press molten plastic through the bore at the same ratethat the rod moves axially of the bore in moving from the first positionto the second position, the rod being provided with a radially-extendingflange which closes the bore when the rod is in the said first position,and an air passage extending through the flange to supply the airpassage in the rod with pressure air when the rod is in the secondposition,

the rod being movable to a third position for stripping a formedarticle, a ring-like stripper being provided with an aperture throughwhich the rod passes in moving from the second position to the thirdposition, and the aperture in the stripper being large enough to allowthe passage of the flange, but not'a portion of the formed article.

2. Apparatus for molding as recited in claim 1, wherein an air passageextends through the rod to a free end, and wherein the said free end islocated in the recess only when the rod is in the said second position.

3. Apparatus for molding as recited in claim 1, wherein the mold isformed in two separable parts with a secondary recess in each halfseparated axially from the first-mentioned recess, wherein the secondaryrecesses serve, when the mold is closed, to clamp around the flange, andwherein a passage extends through the mold into a secondary recess tocommunicate with the air passage in the flange.

4. Apparatus for molding as recited in claim 1, wherein the mold isformed in two separable parts, each part having a wall adjacent theextruder, the walls having mating knife edgesthat meet when the mold isclosed, so that, when the rod has moved to the second position with thefree end within the recess and spaced from the side walls, a tube ofplastic overlies the rod, but a solid cylinder of plastic extends fromthe free end of the rod to the extruder and the closure of the moldcauses the kinife edges to cut through the said cylinder.

5. Apparatus for molding as recited in claim 1, wherein the saidactuator is a fluid cylinder having a piston rod coaxial with andconnected to the blow rod.

6. Apparatus for molding, comprising:

a. an extruder having an outlet bore and a feed means,

b. a mold having a cavity aligned with the bore,

c. a blow rod extending through the mold, and

d. an actuator for bringing about relative movement between therod andbore from a first position in which the rod lies in the bore to a secondposition in which the rod lies in the cavity, and

e. means connecting the actuator and the feed means so that the latteris operated to press molten plastic through the bore at the same ratethat the rod moves axially of the bore in moving from the first positionto the second position, the said bore being formed in a sleeve which isslidably mounted in a housing of the exruder, the said relative movementtaking place by axial movement of the sleeve, and the movement of thesleeve being brought about by the engagement of a rotatable gear mountedin the housing engaging a rack formed on the exterior of the sleeve.

7. Apparatus for molding as recited in claim 6, wherein an air passageextends through the rod to a free end, and wherein the said free end islocated in the recess only when the rod is in the said second position.

8. Apparatus for molding as recited in claim 6, wherein the rod isprovided with a radially-extending flange which closes the bore when therod is in the said first position, and wherein an air passage extendsthrough the flange to supply the air passage in the rod with pressureair when the rod is in the second position.

9. Apparatus for molding as recited in claim 8, wherein the rod ismovable to a third position for stripping a formed article, wherein aring-like stripper is provided with an aperture through which the rodpasses in moving from the second position to the third position, andwherein the aperture in the stripper is large enough to allow thepassage of the flange, but not a portion of the formed article.

10. Apparatus for molding as recited in claim 8, wherein the mold isformed in two separable parts with a secondary recess in each halfseparated axially from the first-mentioned recess, wherein the secondaryrecesses serve, when the mold is closed, to clamp around the flange, andwherein a passage extends through the mold into a secondary recess tocommunicate with the air passage in the flange.

11. Apparatus for molding as recited in claim 6, wherein the mold isformed in two separable parts, each part having a wall adjacent theextruder, the walls having mating knife edges that meet when the mold isclosed, so that, when the rod has moved to the second position with thefree end within the recess and spaced from the said walls, a tube ofplastic overlies the rod, but a solid cylinder of plastic extends fromthe free end of the rod to the extruder and the closure of the moldcauses the knife edges to cut through the said cylinder.

12. Apparatus for molding as recited in claim 6, wherein the saidactuator is a fluid cylinder having a piston rod coaxial with andconnected to the blow rod.

13. Apparatus for molding as recited in claim 6, wherein the said boreis formed in a sleeve which is slidably mounted in a housing of theextruder, wherein the said relative movement takes place by axialmovement of the sleeve, and wherein the'movement of the sleeve isbrought about by the engagement of a rotatable gear mounted in thehousing engaging a rack formed on the exterior of the sleeve.

14. Apparatus for molding, comprising a. an extruder having an outletbore and a feed means,

b. a mold having a cavity aligned with the bore,

0. a blow rod extending through the mold, and

d. an actuator for bringing about relative movement between the rod andbore from a first position in which the rod lies in the bore to a secondposition in which the rod lies in the cavity, and

e. means connecting the actuator and the feed means so that the latteris operated to press molten plastic through the bore at the same ratethat the rod moves axially of the bore in moving from the first positionto the second position,

the rod being movable to a third position for stripping a formedarticle, a ring-like stripper being provided with an aperture throughwhich the rod passes in moving from the second position to the thirdposition, and the aperture in the stripper being large enough to allowthe passage of the rod, but not a portion of the formed article.

15. Apparatus for molding, comprising a. an extruder having an outletbore and a feed means,

b. a mold having a cavity aligned with the bore, and 7 position to thesecond position,

the rod being provided with a radially-extending flange which closes thebore when the rod is in the said first position, and an air passageextending through the periphery of the flange to supply the air passagein the rod with pressure air when the rod is in the second position, andthe mold having an air passage which mates with the air passage of theflange when the mold is in its closed position, to supply air to the airpassage of the flange but the air passage in the mold is spaced from theair passage in the flange when the mold is in its open position.

NI D STATES PATENT OFFICE, 3 CERTIFICATE OF CORRECTION 4 Da ted 7,

Patent No.

Inventor s Johannes Mehriert It is certified that error appears in theabove-identified patent andthat said Letters Patent are hereby correctedas shown below:

7 Oh'the Title page, in item 75 "Sieburg, Germany" should read Menden1iber Sieg berg; Germany and in item 73 the a-ssignee should read SiemagSiegener' Masch'inenbau Gesellsohaft mit besohrankter Haftung,Hilohenbaoh- =D ahlbruoh, Bundes'republik, Germany Signed and sealedthis 17th day ofiDecember 1974.

(SEAL) I 'Attest:

McCOY M. GIBSON J-R.- C. MARSHALL'DANN Attesting Officer I Commissionerof Patents USCOMM'DC 6037 6-P69

1. Apparatus for molding, comprising a. an extruder having an outletbore and a feed means, b. a mold having a cavity aligned with the bore,c. a blow rod extending through the mold, and d. an actuator forbringing about relative movement between the rod and bore from a firstposition in which the rod lies in the bore to a second position in whichthe rod lies in the cavity, and e. means connecting the actuator and thefeed means so that the latter is operated to press molten plasticthrough the bore at the same rate that the rod moves axially of the borein moving from the first position to the second position, the rod beingprovided with a radially-extending flange which closes the bore when therod is in the said first position, and an air passage extending throughthe flange to supply the air passage in the rod with pressure air whenthe rod is in the second position, the rod being movable to a thirdposition for stripping a formed article, a ring-like stripper beingprovided with an aperture through which the rod passes in moving fromthe second position to the third position, and the aperture in thestripper being large enough to allow the passage of the flange, but nota portion of the formed article.
 2. Apparatus for molding as recited inclaim 1, wherein an air passage extends through the rod to a free end,and wherein the said free end is located in the recess only when the rodis in the said second position.
 3. Apparatus for molding as recited inclaim 1, wherein the mold is formed in two separable parts with asecondary recess in each half separated axially from the first-mentionedrecess, wherein the secondary recesses serve, when the mold is closed,to clamp around the flange, and wherein a passage extends through themold into a secondary recess to communicate with the air passage in theflange.
 4. Apparatus for molding as recited in claim 1, wherein the moldis formed in two separable parts, each part having a wall adjacent theextruder, the walls having mating knife edges that meet when the mold isclosed, so that, when the rod has moved to the second position with thefree end within the recess and spaced from the said walls, a tube ofplastic overlies the rod, but a solid cylinder of plastic extends fromthe free end of the rod to the extruder and the closure of the moldcauses the knife edges to cut through the said cylinder.
 5. Apparatusfor molding as recited in claim 1, wherein the said actuator is a fluidcylinder having A piston rod coaxial with and connected to the blow rod.6. Apparatus for molding, comprising: a. an extruder having an outletbore and a feed means, b. a mold having a cavity aligned with the bore,c. a blow rod extending through the mold, and d. an actuator forbringing about relative movement between the rod and bore from a firstposition in which the rod lies in the bore to a second position in whichthe rod lies in the cavity, and e. means connecting the actuator and thefeed means so that the latter is operated to press molten plasticthrough the bore at the same rate that the rod moves axially of the borein moving from the first position to the second position, the said borebeing formed in a sleeve which is slidably mounted in a housing of theextruder, the said relative movement taking place by axial movement ofthe sleeve, and the movement of the sleeve being brought about by theengagement of a rotatable gear mounted in the housing engaging a rackformed on the exterior of the sleeve.
 7. Apparatus for molding asrecited in claim 6, wherein an air passage extends through the rod to afree end, and wherein the said free end is located in the recess onlywhen the rod is in the said second position.
 8. Apparatus for molding asrecited in claim 6, wherein the rod is provided with aradially-extending flange which closes the bore when the rod is in thesaid first position, and wherein an air passage extends through theflange to supply the air passage in the rod with pressure air when therod is in the second position.
 9. Apparatus for molding as recited inclaim 8, wherein the rod is movable to a third position for stripping aformed article, wherein a ring-like stripper is provided with anaperture through which the rod passes in moving from the second positionto the third position, and wherein the aperture in the stripper is largeenough to allow the passage of the flange, but not a portion of theformed article.
 10. Apparatus for molding as recited in claim 8, whereinthe mold is formed in two separable parts with a secondary recess ineach half separated axially from the first-mentioned recess, wherein thesecondary recesses serve, when the mold is closed, to clamp around theflange, and wherein a passage extends through the mold into a secondaryrecess to communicate with the air passage in the flange.
 11. Apparatusfor molding as recited in claim 6, wherein the mold is formed in twoseparable parts, each part having a wall adjacent the extruder, thewalls having mating knife edges that meet when the mold is closed, sothat, when the rod has moved to the second position with the free endwithin the recess and spaced from the said walls, a tube of plasticoverlies the rod, but a solid cylinder of plastic extends from the freeend of the rod to the extruder and the closure of the mold causes theknife edges to cut through the said cylinder.
 12. Apparatus for moldingas recited in claim 6, wherein the said actuator is a fluid cylinderhaving a piston rod coaxial with and connected to the blow rod. 13.Apparatus for molding as recited in claim 6, wherein the said bore isformed in a sleeve which is slidably mounted in a housing of theextruder, wherein the said relative movement takes place by axialmovement of the sleeve, and wherein the movement of the sleeve isbrought about by the engagement of a rotatable gear mounted in thehousing engaging a rack formed on the exterior of the sleeve. 14.Apparatus for molding, comprising a. an extruder having an outlet boreand a feed means, b. a mold having a cavity aligned with the bore, c. ablow rod extending through the mold, and d. an actuator for bringingabout relative movement between the rod and bore from a first positionin which the rod lies in the bore to a second position in which the rodlies in the cavity, and e. means connecting the actuator and the feedmeans so that the latter is operated to press molten plastic through thebore at the same rate that the rod moves axially of the bore in movingfrom the first position to the second position, the rod being movable toa third position for stripping a formed article, a ring-like stripperbeing provided with an aperture through which the rod passes in movingfrom the second position to the third position, and the aperture in thestripper being large enough to allow the passage of the rod, but not aportion of the formed article.
 15. Apparatus for molding, comprising a.an extruder having an outlet bore and a feed means, b. a mold having acavity aligned with the bore, and being movable between a closed and anopen position, c. a blow rod extending through the mold, and d. anactuator for bringing about relative movement between the rod and borefrom a first position in which the rod lies in the bore to a secondposition in which the rod lies in the cavity, and e. means connectingthe actuator and the feed means so that the latter is operated to pressmolten plastic through the bore at the same rate that the rod movesaxially of the bore in moving from the first position to the secondposition, the rod being provided with a radially-extending flange whichcloses the bore when the rod is in the said first position, and an airpassage extending through the periphery of the flange to supply the airpassage in the rod with pressure air when the rod is in the secondposition, and the mold having an air passage which mates with the airpassage of the flange when the mold is in its closed position, to supplyair to the air passage of the flange but the air passage in the mold isspaced from the air passage in the flange when the mold is in its openposition.